2 different shader nodes for fresnel reflectance of conductive materials.
One has a typical interface for creating conductive materials, input your n, k values manually.
Other for more artistic use, has interface for setting Reflective and EdgeTint colors: http://jcgt.org/published/0003/04/03/

Light reflecting off metallic surfaces is described by the Fresnel equations [Born and Wolf 1999], which are controlled by the complex index of refraction η = n + ik.

Together, n and k determine the two characteristics of the Fresnel curve for a material: the reflectivity at normal incidence and how quickly it fades to white at grazing angles. However, this parameterization presents some artistic challenges because both characteristics depend on both parameters. An artist would ideally manipulate each property independently and have them on a unit scale. This paper describes a remapping for the approximated unpolarized Fresnel equations of n and k to the more intuitive reflectivity, r, and edgetint, g, both in the range from 0 to 1.